1. Field of the Invention
The present relates to an organic photovoltaic element such as a solar battery, which can be applied to an optical sensor and the like.
2. Discussion of Background
Many organic photovoltaic elements using as an active material an organic material have been studied in order to develop an organic photovoltaic element which is inexpensive and free from toxicity. This is because it is extremely difficult to develop such an organic photovoltaic element by use of single crystalline silicon, polycrystalline silicon, or amorphous silicon.
A photovoltaic element is an element which can convert light energy to electrical energy (voltage x current). Therefore the photovoltaic element is evaluated mainly by the conversion efficiency of light energy to electrical energy. An internal electric field must exist for the generation of photocurrent. Several elements with particular structures for generating such an internal electric field have already been known. The optimum conversion efficiencies of the conventionally known elements, obtained by use of organic materials as the active material, are as follows:
(1) Schottky junction or MIS type junction:
In this junction, an internal electric field generated at a metal/semiconductor junction is utilized. As organic semiconductor materials for the above junction, for instance, merocyanine dyes and phthalocyanine dyes have been reported. For instance, in the case of an Al/merocyanine dye/Ag element, the conversion efficiency is 0.7% (V.sub.OC =1.2 V, J.sub.SC =1.8 mA/cm.sup.2, ff=0.25) with the irradiation of a white light of 78 mW/cm.sup.2 (A. K. Ghosh et al., J. Appl. Phys. 49, 5982 (1978)). Organic semiconductors which are employed in this type of photovoltaic element and have high conversion efficiencies are limited to p-type organic semiconductors. Therefore, materials employed for the electrodes are limited to those with a low work function, such as Al, In, and Mg. These materials, however, have the drawback that they are easily oxidized.
(2) Hetero p-n junction utilizing an n-type inorganic semiconductor/p-type organic semiconductor junction:
In this junction, an internal electric field generated at an n-type inorganic semiconductor/p-type organic semiconductor junction is utilized. As the n-type inorganic semiconductor, for instance, CdS and ZnO are employed, and as the p-type organic semiconductor, for instance, merocyanine dyes and phthalocyanine dyes have been reported as being usable.
An optimum conversion efficiency obtained so far is 0.22% (V.sub.oc =0.69 V, J.sub.sc =0.89 mA/cm.sup.2, ff=0.29) by use of an ITO/electrodeposited CdS/chlorinated aluminum chlorophthalocyanine/Au element with an AM-2 irradiation of 75 mW/cm.sup.2 (A. Hor et al., Appl. Phys. Lett., 42, 15(1983)).
(3) Organic/organic hetero p-n junction:
In this junction, an internal electric field generated by the junction of an organic electron acceptor material and an organic electron donor material is utilized.
Examples of such an organic electron acceptor material are dyes such as Malachite Green, Methyl Violet, pyrylium compounds, and fused polycyclic aromatic compounds such as flavanthrone, and perylene pigments.
It is reported that the conversion efficiency of an ITO/copper phthalocyanine/perylene pigment/Ag element was 0.95% (V.sub.oc =0.45 V, J.sub.sc =2.3 mA/cm.sup.2, ff=0.65) with an AM-2 irradiation of 75 mW/cm.sup.2 (C. Tang. Appl. Phys. Lett., 48, 183 (1986)). This value of the conversion efficiency is the largest obtained by an organic photovoltaic element using an organic material. In Japanese Patent Publication 62-4871 in the name of C. Tang, it is reported that a conversion efficiency of 1.0% (V.sub.oc =0.44 V, J.sub.sc =3.2 mA/cm.sup.2, ff=0.6) was obtained when a perylene pigment different from the above-mentioned perylene pigment was employed in a photovoltaic element with the same structure as mentioned above.
Generally the conversion efficiency of organic photovoltaic elements is lower than that of inorganic photovoltaic elements. The most significant factor for this difference is that organic photovoltaic elements have a low short-circuit photocurrent (J.sub.sc). At least a short-circuit photocurrent (J.sub.sc) of 10 mA/cm.sup.2 is necessary for a photovoltaic element with a conversion efficiency of 5%, with a white light irradiation of 75 mW/cm.sup.2. However, the photocurrent (J.sub.sc) of any of the above-mentioned organic photovoltaic elements is much lower than 10 mA/cm.sup.2. This is caused by the low quantum efficiency of the organic photovoltaic elements and by the narrowness of the wavelength range of spectral sensitivity thereof. It is desirable that the wavelength range of spectral sensitivity be extended from 400 nm to a larger range as much as possible. However, the conventional organic photovoltaic elements mentioned above have a very limited wavelength range of spectral sensitivity.
Furthermore, many conventional organic photovoltaic elements have a small ff value. It is said that one of the reasons for the small ff value is that the quantum efficiency of any of the organic semiconductors employed therein is rapidly decreased in a low electric field. Therefore, it is preferable to employ an organic photovoltaic element having such a structure as to generate an internal electric field with high intensity in order not to decrease the quantum efficiency of the organic semiconductor employed therein. Furthermore, a photovoltaic element having a structure that can cause generated electric charges to smoothly reach the electrodes without energy barriers increases the ff value. If these can be attained, the value of V.sub.oc can be increased, but the above-mentioned factors are not taken into consideration in many conventional organic photovoltaic elements.
In addition, many conventional organic photovoltaic elements have problems with respect to the chemical stability of the materials employed for electrodes.
From this point of view, the following conventional organic photovoltaic elements will now be explained.
(1) Schottky junction or MIS junction:
In this junction, a large value of V.sub.oc can be obtained. However, since metallic materials are used for the electrodes, the light transmittance of the electrodes is low. The light transmittance obtained in practice is 30% at best, usually about 10%. Furthermore, the metallic materials used for the electrodes are poor in resistance to oxidation. Therefore, it is not expected to fabricate an organic photovoltaic element with high conversion efficiency and stable characteristics by use of a Schottky junction or a MIS junction.
(2) Inorganic semiconductor/organic semiconductor hetero p-n junction:
Electric charges are mainly generated in the organic layer, so that a photovoltaic element using this junction is restricted with respect to the spectral sensitivity thereof. Usually, the organic layer in the photovoltaic element is composed of one organic semiconductor. However, there are no organic semiconductors with a high light absorption property capable of absorbing light in a wavelength range of, for instance, from 400 nm to 800 nm. A photovoltaic element with the structure using this junction can solve the problems concerning the light transmittance of a light-incident electrode and the stability of the electrodes, but the region of the spectral sensitivity is so narrow that a high conversion efficiency will not be obtained.
(3) Organic/organic hetero p-n junction
This junction is better than the above-mentioned two types of junctions and the most desirable junction obtained at present. Light irradiation can be carried out through a transparent electrode, and photoelectric charges can be generated by use of two organic materials, so that the range of the spectral sensitivity can be expanded. As a matter of fact, according to the previously mentioned Tang's report, perylene pigments can generate electric charges with the irradiation of light having a wavelength of 450 to 550 nm, and copper phthalocyanine can generate electric charges with the irradiation of light having a wavelength of 550 to 700 nm. Furthermore, the ff value obtained by an organic photovoltaic element using this junction is larger than that obtained by other photovoltaic elements. It is considered that this is because the intensity of the internal electric field generated therein is larger than that of the internal electric fields generated in other photovoltaic elements.
However, Tang's photovoltaic element has the following drawbacks:
(1) In his patent, it is described that it is preferable that the organic layer have a thickness of 300 to 500 .ANG.. However, the organic layer is so thin that there is a high probability that pin holes are formed therein. Our experiments demonstrated the occurrence of short-circuits circuits between two electrodes with a relatively high probability, which is considered to be caused by the presence of pin holes in the organic layer. According to C. Tang's paper, the area of the electrodes is 0.1 cm.sup.2. However, when the area is increased to 1.0 cm.sup.2 or more for use in practice, the improvement of the yield of the photovoltaic element without the defects caused by the pin holes will become a big problem.
(2) C. Tang's invention has a problem in the materials for the electrodes. In his invention, it is necessary that each of the electrodes be in ohmic contact with the respective counterpart organic layer. In C. Tang's previously mentioned paper (Appl. Phys. Lett., 48, 183 (1986)), it is described that when the overlaying order of the organic layers is reversed in the photovoltaic element, the value of V.sub.oc is decreased. It is considered that this is caused by impairment of the ohmic contact. On the other hand, in the structure in which the ohmic contact is attained, the stability of the metallic materials for the electrodes becomes a problem because it is necessary that the metallic materials which come into contact with organic electron acceptor materials have a low work function. In C. Tang's Patent, In, Ag, Sn, and Al are given as examples of such metallic materials. However, these materials are easily oxidized.